docs: def deriv(self, n=1):
          """ Compute *n*-th derivative of ``self``.
  
          :param n: Number of derivatives.
          :type n: positive integer
          :returns: *n*-th derivative of ``self``.
          """
          if n==1:
              if self.order > 0:
                  return PowerSeries(self.c[1:]*range(1,len(self.c)))
              else:
                  return PowerSeries([0. * self.c[0]])
          elif n>1:
              return self.deriv().deriv(n-1)
          elif n==0:
              return self